PAPI & APAPI COMPLETE NOTES

WHAT ARE PAPI AND APAPI?

PAPI (Precision Approach Path Indicator) and APAPI (Abbreviated PAPI) are visual approach slope indicators. Aimed red and white beams tell the pilot whether the aircraft is above, on or below the approach slope. They are not radio aids like an ILS glide slope.

FAA AC 150/5345-28H classifies the four-unit system as L-880 and the two-unit system as L-881. ICAO likewise defines PAPI as a four-unit wing bar and APAPI as a two-unit wing bar, normally on the left side of the runway unless impracticable.

Suitability is set by the aerodrome requirements, aircraft, approach design and project specification. PAPI can be harmonised with an ILS but does not replace it.

THE LIGHT SIGNALS

PRINCIPLE: THE RED/WHITE TRANSITION

Each unit projects one beam split horizontally: aviation white above, aviation red below. The boundary is the transition band, and AC 150/5345-28H defines the beam centre as that transition. As the aircraft changes angle it crosses each unit's transition, so the pilot sees a different white/red mix.

Viewed at 1,000 ft (300 m), the transition must be within 3 minutes of arc at beam centre and 5 minutes at the beam edges. Colours must meet SAE-AS25050 and, for LED systems, FAA Engineering Brief #67.

APPROACH PATH, TCH, EYE-TO-WHEEL AND OCS

The visual approach path is normally 3° (FAA Order 6850.2C) but is set by the approved aerodrome design; do not assume 3° everywhere. Slope: θ = arctan(H / D).

LAYOUT AND UNIT ORDER

Units form a straight line (wing bar) perpendicular to the runway edge. In a left-side installation, unit D is nearest the runway (inboard) and unit A is farthest.

RUNWAY | [D] [C] [B] [A]
         inboard      outboard

Use the approved project siting document rather than spacing from memory. ICAO allows small height adjustments between units to suit transverse slope.

AIMING ANGLES

The units are deliberately aimed at different angles around the selected path. If all were aimed alike you would not get the stepped red/white sequence.

Other arrangements use wider offsets, for example 3°35′, 3°15′, 2°45′, 2°25′ on a 3° path (+35′, +15′, −15′, −35′), which ICAO-based documents give for some ILS-harmonised or steeper-slope designs. The 4-unit offsets above are the usual FAA and ICAO standard for a 3° path. These are not universal settings: the approved design, and the standard it follows, decides.

Built-in adjustment covers 2° to 8°, and the manufacturer's aiming tool reads in 10-minute increments with about ±3′ repeatability. Aiming is a safety-critical adjustment: errors change the pilot indication, TCH, obstacle clearance and ILS agreement.

OPTICS AND CONSTRUCTION

ELECTRICAL: STYLE A AND STYLE B

Type (L-880/L-881) counts the units, Style (A/B) is the powering method, and Class (I/II) is the temperature range. The L-828 regulator and isolation transformer are not supplied with a Style B PAPI and must be compatible.

Style B specifies Class A Style 1 or Style 6 plugs under AC 150/5345-26 to mate with the transformer output lead (see Note 10, Note 08 and Note 09). In both styles a lamp failure must not damage the power supply or the remaining lamps; for Style B a lamp bypass (shorting device) is available on request.

CONTROL AND DAY/NIGHT OPERATION

A Style A PCU may be separate or integrated and can hold power supply, control, intensity control, monitoring and protection. Style A requirements include full-intensity day mode, automatic photoelectric switching with hysteresis and a delay of about 45–75 s, defaulting to night mode if the photocell fails, night modes of roughly 5% and 20% of day intensity, and remote ON/OFF by hardwire or radio. An optional go/no-go lamp-monitor output can report status. Wrong intensity causes poor visibility, glare or reduced contrast.

SITING AND INSTALLATION

Survey data (see FAA Engineering Brief #95 for additional siting and survey considerations): runway centreline and threshold elevation, PAPI foundation elevation, longitudinal and transverse slope, obstacles, ILS reference, aircraft group and existing visual aids. A longitudinal gradient means siting cannot assume flat ground; distance, elevation and aiming may need to change.

PHOTOMETRY AND MAINTENANCE

"All lamps ON" does not mean "PAPI compliant." A PAPI can be lit yet have wrong intensity, transition, aiming or colour. Photometry checks intensity, beam distribution, chromaticity, transition quality, beam geometry and horizontal coverage.

Tools: precision inclinometer and aiming tool, survey level, true-RMS and clamp meters, insulation tester where applicable, photometric equipment, torque and cleaning tools, all calibrated where required. Record designed vs actual angle, difference, elevation, alignment and condition per unit, and keep a maintenance log (date, runway, unit, fault, action, final angle, test result, technician, return to service). Documentation should include the instruction book, drawings, aiming and calibration procedures, FAT and qualification reports and photometric data.

ELECTRICAL SAFETY

FAULT-FINDING

Before changing an angle, check the approved angle, survey reference, unit level, runway and PAPI elevation, TCH, aircraft group, ILS relationship, OCS and the manufacturer's procedure; not just "it looks high".

COMMON MISTAKES AND COMMISSIONING CHECKLIST

Commissioning checklist

QUICK REVISION

PAPI
 ├── L-880 (4 box): 4W → 3W1R → 2W2R → 1W3R → 4R
 └── L-881 (2 box): 2W → 1W1R → 2R
STYLE A: utility -> PCU -> PAPI
STYLE B: CCR -> series -> isolation transformer -> PAPI

EXAM / INTERVIEW QUESTIONS

FINAL STUDY TAKEAWAY

PAPI tells the pilot high, on slope or low by changing the ratio of white to red through precisely aimed beams. Understand it as optics + geometry + electrical + mechanical + photometry + maintenance.

ENGINEERING & EXPERT NOTES

REFERENCES & FURTHER READING

These notes are a study aid. Verify details against the primary sources below.

EXPLORE ALL AGL STUDY NOTES